The structure and function of human wt p53 have been investigated in order to understand the anti-proliferation effect of wt p53 on cell growth. Analysis of the effect of wt p53 on cell-cycle and growth regulated gene expression revealed that expression of wt p53 did not affect expression of immediate early genes such as fos, myc and jun but did down-regulate transcription of several late Gl genes, b-myb, PCNA and DNA polymerase alpha. These results, together with our observation that p53 arrests cells in Gl prior to the restriction point, indicate that wt p53 acts to arrest cells at a specific point (mid to late Gl phase) in the cell cycle. Biochemical analysis of p53 found that growth arrest mediated by wt p53 correlated with a unique conformational and phosphorylation state of wt p53 compared to mutant p53. The increased phosphorylation was in the N-terminus of the protein and preliminary evidence suggests that it involves ser 15, a site phosphorylated by the DNA-activated protein kinase, as well as one or two additional serine residues. Site directed mutagenesis of this residue suggested that ser 15 is involved in regulation of p53 turnover.